The invention relates to liquid crystal phases (LC phases) with low optical anisotropy and wide nematic phases.
The properties of nematic or nematic-cholesteric liquid crystal materials are being increasingly utilized for liquid crystal display elements (LC display elements) to significantly modify their optical properties, such as light absorption, light scattering, birefringence, refractance or color under the influence of electrical fields. The function of such display elements here is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases, the guest-host effect, the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transfer.
Liquid crystal phases which must fulfil a large number of requirements are required for industrial application of these effects in electronic components. Chemical stability towards moisture, air and physical influences, such as heat, infrared, visible and ultraviolet radiation and constant and alternating electrical fields, are particularly important here. Furthermore, a liquid crystal mesophase in a suitable temperature range and the lowest possible viscosity at room temperature are required of industrially usable liquid crystal dielectrics. Finally, they should not exhibit intrinsic absorption in the range of visible light, that is to say they must be colorless.
In none of the previously known series of compounds with liquid crystal mesophases is there an individual compound which meets all these requirements. Mixtures of two to twenty, preferably three to fifteen, compounds are therefore as a rule prepared in order to obtain substances which can be used as liquid crystal phases. At least one compound of low melting point and clear point is usually included in this mixture. A mixture is usually obtained here with a melting point below that of the component of lower melting point, while the clear point is between the clear points of the components. However, optimum phases cannot easily be prepared in this manner, since the components of high melting point and clear points frequently also impart a high viscosity to the mixtures. The switching times in the electrooptical display elements produced with these mixtures are thereby increased in an undesirable manner.
The invention is based on the object of preparing liquid crystal phases which have a nematic phase in the required temperature range and allow adequately short switching times in liquid crystal cells at room temperature.
A large number of liquid crystal phases based on a large number of compounds are already commerically available. However, there is still a great need for liquid crystal phases with high clear points, lower melting points, low viscosity (and hence short switching times) and low optical anisotropy. Liquid crystal phases with a wide mesophase range, low viscosity, low optical anisotropy, steep characteristic transmission line and relatively highly negative dielectric anisotropy are particularly required for so-called guest-host displays with positive contrast (the information appears in dark form on a light background) (T. J. Scheffer, Phil. Trans. R. Soc. Lond. A 309 (1983) 189; and F. Gharadjedazki and R. Voumasd, J. Appl. Phys. 53 (1982) 7306). Moreover, the liquid crystal phases must have a high stability to UV and take up the dichroic dyestuffs to a sufficient degree. Such negative LC phases were not hitherto available. For wide-range mixtures, for example for external applications, the product of the layer thickness and optical anisotropy must have quite specific values, for example about 1.0 or about 0.5, since otherwise the angle-dependency of the contrast becomes too great and interference colors arise, impairing the optical phenomenon. A coating thickness of about 6-7 micrometers and a liquid crystal phase with a positive dielectric anisotropy and an optical anisotropy of about +0.07 to about +0.08 are today generally employed for such TN cells in order to achieve a particularly good angle-dependency of the contrast. It is particularly difficult to achieve sufficiently high clear points while at the same time avoiding the occurrence of smectic phases and viscosity values which are too high at low temperatures. Such positive LC phases have not hitherto been available.